Abstract
Future lightwave communications systems are envisioned where the optical waves are treated like present-day radio waves, allowing angle modulation, heterodyne detection, and many multiple channels through frequency multiplexing. One essential device for such systems is the analog of the crystal oscillator to provide an absolute frequency reference in the optical domain. A simple and practical approach to obtain these devices wall be the use of frequency-stabilized semiconductor lasers. To ensure long- term stability, atomic or molecular absorption lines can be used as external frequency references. However, most previous work on absolute stabilization of semiconductor lasers has been done in the 0.8-μm region.1 In the 1.3- and 1.5-μm regions,2-4 where conventional silica-based optical fibers show low dispersion and low loss, experiments using the generally more complex and faint molecular spectra have required long absorption cells due to low absorption coefficients. Because of the lack of any useful atomic transitions originating in the ground state, the optogalvanic effect using the excited-state transition has been used to stabilize the frequency of semiconductor lasers at 1.3- and 1.5-μm.5,6 The technique using the optogalvanic effect is of particular interest because of its simplicity, compactness, high ratio of signal strength to laser power, and wide choice of reference lines within the telecommunication window. Table 1 summarizes many atomic transitions available in 1.3- and 1.5-μm regions. (Optogalvanic responses of these transitions have been surveyed experimentally.7) These lines can be excellent frequency references because noble gases are relatively unsusceptible to external perturbations. In addition, the signals are strong and well separated, thus facilitating their use in lightwave communications systems.
© 1991 Optical Society of America
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